Control apparatus



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I NVEN TOR. MARLOWE W. NERSON ATTORNEY Oct. 29, 1968 M. w. IVERsoNCONTROL APPARATUS 5 Sheets-Sheet 5 original Filed Feb. 2, 1962' ONN\AATTORNEY 5 Sheets-Sheet 4 EET-124 mnEDm M. W. IVERSON CONTROL APPARATUSINVENTOR. MARLowE w. NERsoN ATTORNEY w n H M x Ww 1 f8. v new n 1S? 1 uwy )El L i Nw :ESO *"w Nm EIL/gm 1 Oct. 29, 1968 Original Filed Feb. 2,

Oct. 29, 1968 M. w. IVERSON CONTROL APPARATUS 5 Sheets-Sheet 5 OriginalFiled Feb. 2,

ATTORNEY United States PatentN O ABSTRACT F THE DISCLOSURE Astabilization system for an aircraftutilizes two corn- I pletelyseparate active chanels; each channel includes two servomotors. Oneservomotor of each channel is connected in a series (often termeddifferential) arrangement with a servomotor of the other channel, theoutput of the vseries arrangement --operates a control surface of anaircraft. Thus, for example, two servomotors are connected, for each ofthe two rudders or other control surfaces of the craft, in adifferential or whifiie-tree arrangement. A third or monitoring channelduplicates the sensors and the electronics employed in the first twochannels, and the electronics in the three channels provide a majorityvoting system for rendering ineffective a malfunetioning channel, Thus,in the event of a failure in one active channel or Vthe other activechannel, a comparator or decision device of the monitoring arrangementdisengages the faulty channel and concurrently raises the gain of theremaining active channel to compensate for the narrowed or reducedmechanical gain through the differential or whiflle-tree arrangement.The identity of a failed active channel is obtained in the decisiondevice. Therein the electronics of one active channel may be comparedwith that of the other active channel, and also separately compared withthe electronics of the monitor channel. Thereafter, the resultanteffects of the two separate comparisons are applied to a logic devicefor disengaging the one active channel upon malfunction thereof. Whenthe malfunctioning channel is disengaged, the remaining active channelhas the gain therein increased. t

This application is a division of application S.N. 170,- 695, filed Feb.2, 1962, now U.S. Patent No. 3,351,315.

This invention pertains to control apparatus for controlling a conditionand more particularly relatesA to a stability augmentation system forcontrolling an aircraft in flight about one of its control axes.

While stability augmentation systems have been heretofore provided, anobject of this invention is to provide an improved augmentation systemwith a high degree of reliability.

A further object of this invention is to provide an irnprovedaugmentation system wherein the reliability improvement is achievedusing redundant channels.

A further object of this invention is to provide in a redundant channelstability augmentation system an automatic monitor effective on failureof a channel to delete or render ineffective for control purposes ofthevcraft the failed channel.

A further object of the invention is to permit the pilot of the aircraftto override the automatic monitor under certain failure conditions.

v The above and further objects and advantages of my invention willappear hereinafter from a consideration of the followingdescription.andaccompanying drawings illustrating a preferred embodimentthereof.

` "In the drawings:

FIGURE 1 is a block diagram of a redundant channel stabilityaugmentation system applied to the yaw axis of an aircraft;

3,408,024 Eatented Oct. 29, 1968 ICC FIGURE 2 is a block diagram of themonitoring apparatus of the system of FIGURE l for controllingdisengagement of a failed channel;

FIGURE 3 is an electrical schematic of a portion of the` redundantchannel stabilization system of FIGURE l; and

, FIGURES 4 and 5 comprise an electrical schematic of the monitoring anddisengage arrangement of FIGURE 2.

While I have shown my invention applied for purposes of illustration tothe yaw channel of an aircraft, it is to be understood that it is alsoapplicable either to the pitch channel or to the roll channel of thecraft or to control systems generally using a condition control device.The yaw axis control system consists of a dual channel yaw stabilityaugmentation system and a third or monitor channel wherein there areduplicates of the gyro, accelerometer, and electronics ofthe other twochannels. For purposes rof identification, `one dual channel is referredto as the A channel, the other as the B channel and the monitor channelis referred to as the M channel;

The yaw axis stabilization system herein utilizes two completelyseparate active channels operating on four servomotors connected in aseries (often termed differential) arrangement.

Thus two servomotors are connected, for each of the two rudders of thecraft, in a differential or Whittle-tree arrangement. A third ormonitoring channel duplicates the rate gyro, the lateral accelerometer,and the electronics employed in the first two channels, to provide avoting system.

In the event of a failure in channel A or channel B, the comparator ordecision device of the monitoring arrangement disengages the faultychannel and concurrently raises the gain of the remaining active channelto compensate for the narrowed or reduced mechanical Vgain through thedifferential or whiie-tree arrangement.

Provisions are included in the system to permit the pilot, following theoccasion of a failure, to reset or recycle the apparatus if a temporaryfailure only has occurred.

Referring to FIGURE l, rudder pedals such as rudder pedal 12 in anaircraft and which pedal is pivoted on the craft at 13 operates througha connecting link 14, three armed lever 15, link 17, bell crank lever18, link 19, differential lever 20 pivoted at 26 to position an -outputmember 24 pivoted at an intermediate point 25 on the differential lever20. The output member 24 may be used to position directly or through aboost actuator a right rudder of an aircraft.

Operation of three armed lever 15 is also transmitted through link 30,bell crank lever 31, pivoted on the craft at 32, link 33, differentiallever 34, onpivot 35, having an intermediate point 36 thereof from whichextends a second output member 37 which may directly or through a boostactuator position a left rudder surface of an aircraft. The operation oflink 19 is opposed by a Bungee apparatus 38 on the aircraft andsimilarly the operation of link 33 is resisted by a Bungee arrangement39 on the craft. By means of a switching arrangement 40 having a pilottrim operable arm 41, a trim actuator 42 and Va trim actuator 43 may beoperated to alter the normal biasing point of links 19 and 33respectively.

Coa'cting with the differential lever 20 at pivot 26 through its outputlink 45 is one series servo arrangement 44, and coacting withdifferential lever 34 at pivot 35 through its output link 47 is a seconddifferential or series servomotor arrangement 48, so called becauseoperation of links 45 or 47 is not reflected back as to cause movementof pedal 12.

Differential arrangement 44 comprises a first hydraulic or fluidservomotor 50, a second hydraulic servomotor 51 with the servomotorhavin-g their respective output members 52, 53 connected to opposed endsof a link 54 which in turn has its midpoint connected to link 45 eX-tending to pivot 26 on differential lever 20. The servomotor 50 isbiased -to normal centered position through an internal spring centeringarrangement illustrated for simplification here as -arrangement 56, andsimilarly the servomotor 51also when rendered ineffective is biased to anormal centered position and locked therein by an internal springcentering arrangement represented herein by arrangement 57.

Differential servomotor arrangement 48 comprises a hydraulic servomotor60, a hydraulic servomotor 61, the hydraulic servomotors having theirrespective output members 62, 63 connected to opposed ends of a link 64havin-g an intermediate point thereof connected to link 47 extending topivot 35 on differential lever 34. The servomotors 60, 61 similarly tomotors 50, 51 have respective centering means 66, 65 for centering andlocking the servomotors upon such servomotor being rendered ineffectivefor operation.

Each of the servomotors is rendered effective for control operation byan electrohydraulic engage valve (EHE) upon energization thereof. Thusservomotor 50 has an engage valve 58, motor 51 has an engage valve 59,motor 60 has an engage valve 67, and motor 61 has an engage valve 68.When the servomotors are rendered effective for operation, suchoperation is controlled by their respective amplifiers 71, 72, 73 and74.

The general type of hydraulic servomotors, the means for rendering eachservomotor effective, and the means such as transfer valves (TV) shownfor controlling operation of such servomotor when rendered effective foroperation as thus far described are old in the art and may be of thetype disclosed in the patent to Schurr No. 2,952,- 424.

While the patent shows a different arrangement of the servomotor havingservomotor 14 therein associated with a manually controlled end 12, itis to be understood that in the present use of a servomotor of this typeonly the autopilot section 14 will be utilized. Furthermore the body orhousing of such servomotor instead of being pivoted on the craft as atin the patent will be fixed relative to the craft for rectilinearmovement of its output member such as the piston rod. Such servomotor asin the patent has a solenoid operated valve 65 for preconditioning orrendering the servomotor effective for subsequent control or oper-ation.When valve 65 is deenergized, spring 55v centers and locks the servooutput member 16. The operation of servo 14 is achieved through thepositioning of a pilot valve 40 from a normal position. Valve 40, whichmay be considered a transfer valve is differentially positioned in onedirection or the other by the output from an amplifier. Such amplifieras 72 of the patent corresponds with the amplifiers 71, 72, 73 and 74herein.

Amplifier 71 receives a control signal from conductor 75 extending froma summing device 76. Device 76 provides a resultant signal obtained fromone signal on -conductor 78 representing responses of the craft tochanges while in flight and a servo displacement signal on transmittingmeans 79 received through a -gain device 80 from displacement of theoutput member 52 of servomotor 50. Similar summing devices 81, 82 and 83and sources of control signals are provided for amplifiers 72, 73 and 74respectively.

It will be noted that the same input signal supplied by conductor 78 tosumming device 76 is also supplied Via subconductor 93 to summing device82. Consequently there is ya pair of servomotors, as 50 and 60, in eachdifferential arrangement 44, 48 controlled by the same input controlsignal.

The signal on input conductor 78 which corresponds with the signalderived from the response of the craft to various flight conditions isin turn derived from a conductor 90 extending from a summing device 91.

Summing device 91 supplies to conductor 90 an output signal from achannel A of the yaw stability augmenter apparatus. Channel A comprisesa yaw rate sensing device 97 such as a conventional yaw rate responsivegyroscope which provides on conductor 98 an alternating signal voltageof a magnitude dependent upon the rate of yaw of the craft end of aphase depending upon the direction of yaw of the craft. The AC signal onconductor 98 is supplied through a conventional diode limiter 99 whichlimits the magnitude of the signal transmittedover conductor 100 to anamplifier-demodulator 101. The AC input is thereby converted to a kDCoutput and supplied to a summing device 102. Summing device 102 undercircumstances to be described additionally receives the output of asecond amplifier-demodulator 103 which is connected at times throughswitching arrangement 104 to conductor 100.

Thearrangement of the two vampliliendemodulators 101 and 103 in parallelwith switching arrangement 104 in the opposite position from that shownis representative of a doubling in gain of amplifier-demodulator 101upon disengagement of either active channel, to be described.

The output `from summer device 102 is supplied through a gain controldevice 106 having the DC output therefrom supplied by conductor 107 to ahigh pass network 108 which blocks con-t-rol signals of low frequency.The output from network 108 is supplied in turn to an yaeroelasticfilter 110 represented as a lag device and the output from theaeroelastic filter is supplied to summing device 91.

Channel A also includes an accelerometer sensing lateral accelerationsof the craft which derives an AC signal of a magnitude in accordancewith the magnitude of the lateral accelerat-ions of the craft and of aphase depending upon the direction of acceleration. The output fromlateral accelerometer 112 is transmitted through conductor 113 toamplifier-demodulator 114 having its DC output supplied to summingdevice 115 which in turn through gain Control device 118, passive lagdevice 120,'and couductor 121 supplies the other input to summing device91. The output of the device is a composite signal identified as XrbA.

The yaw stability augmenter additionally includes as a source of controlsignals ya channel B having a yaw rate sensor and lateral accelerometersensor similar to channel A and having the elements following such yawrate sensor and lateral accelerometer corresponding to channel A so thatthe control signals are supplied to summing device 125. The compositesignal Xr'B from device 125 is -t-ranslrnitted through conductor 126 tosumming arrangement 81 which controls servomotor amplifier 72 and whichis also supplied through summing device 83 which through servoamplifier74 controls servomotor 61.

As is thus evident that a servomotor 51 in differential arrangement 44and a servomotor 61 in differential arrangement 48 are also controlledby a common resultant aircraft flight condition electrical controlsignal.

As thus far described, the apparatus provides a stability laugmentationsystem for an aircraft in yaw, 'but the primary features of theinvention pertain to a monitoring arrangement for such augmentationsystem. The description of such monito-ring arrangement will now beconsidered. The stabilization system includes a third channel Msignifying a monitoring channel. This channel duplicates the yaw ratesensing deviceand the lateral accelerometer sensing device plus theassociated electronics included in channels A and B previouslydescribed. The arrangement issuch that at a summing device 130 in theMchannel there is ya resultant control signal XrM` applied to conductor131.

It will be noted that there are three devices sensing the same aircraftflight condition such vas yaw rate and three flight condition sensingdevices sensing a second flight condition such as lateral acceleration.Since each channel thus duplicates these members as well as followingelements, the outputs from summing devices 91 and 125 and 130 shouldordinarily be the same. The output from summing device 91 as stated isreferred to as (Xgl/A); the output from summing device 125 is referredto -as (XrbB); and the output from summing device 130 is referred to as(XiM). Also the displacement of servomotor 50 derived from the servo`displacement followup signal conductor 79 represented as (XAR) shouldbe the same as the electrical signal derived from operation of theservomotor 60 appearing on servo displacement followup conductor 69 andrepresented as (XAL) since the two servomotors 50, 60 receive a commoncontrol signal. Also, the electrical followup signal (XBL) on servofollowup conductos 134 of servomotor 61 should be the same as theelectrical signal (XBR) on followup conductor 135 of servomotor 51. Theabove seven signals are utilized in a monitoring arrangement shown inblock form in FIGURE 2.

In FIGURE 2 there are shown ivesum-ming devices 150, 151, 152, 153 and154. Summing device 150 receives servo displacement signals XAL and XARand the algebraic signs of these signals are always in opposition. Theoutput from summing device 150, if any, is applied through conductor 156to buffer amplifier 157, which may be a two-stage transistor amplier.Ordinarily the two opposed signals to device 150 are equal, and thus noinput control signal or error signall is supplied on conductor 156.

The output from amplifier 157 is supplied via conductor 158 to abistable device here called a memory circuitor flip iiop 159. Devices ofthis type are bistable, and they are often used in high speed countingdevices. They may be either of the transistorized type or the vacuum-tube type. The output from the memory device 159 which isga DC voltagelevel is supplied over conductor 161 to a switching logic, or gate, `asindicated by the G5 symbol. Upon such actu-ation of the 0r gate 162dueto an enror signal its operation is applied through conductor 163 toa power switch 164 controlling `a main engage circuit. Thus upon anoutput of such type from 0r gate 162 due to the error signal, theservomotors AL and AR are rendered inelective, centered and locked ordisengaged. In other words the A channel is disengaged, and indicatingmeans such as a function selector A light (not shown) and an aircraftmaster warning light (not shown) are energized. The monitoringarrangement thereby shows a 'faulty operation or malfunction in the Achannel.

Similarly the summing devi-ce 151 has applied thereto the B channel leftand right servo displacement signals, and if there be a difference orerror signal, the dilference is applied over a conductor 170 in ltheform of an electrical signal to buter amplifier 172. Amplifier 172 inturn has its output supplied over conductor 173 to the bistable deviceof ip op (FF) 174. The output from the device 174 which is in the formof a DC voltage level due to the error signal is applied over conductor177 to a second or switching logic 179. The output from the or switchinglogic 179 is supplied over conductor 180 to power switch 181 causingdisengagement of the B channel servomotors, in a manner moreparticularly to be described.

While the monitoring arrangement as thus far described in FIGURE 2effects the monitoring of the displacement of the four Seriesservomotors operating the two rudder surfaces of an aircraft, in thefollowing, the flight condition sensors and electronics which sense theflight conditions of anaircraft and supply cont-rol signals aremonitored. In the sensor monitoring arrangement vas `distinct from theservo operation monitoring arrangement, summing device 152 has appliedthereto for simple terminology an A channel composite control signal anda B channel composite control signal of opposite polarity and anydilerence in these signals when algebraically summed is supplied overconductor 190 to amplier 191. Amplifier 191 has its output supplied byconductor 192 to bistable or memory circuit or flip op device 193 whichin turn through conductor 194 has its output supplied to a switchinglogic and device 196 yrepresented by the symbol By means of asubconductor 197 extending from conductor 194, the output of memorydevice 193 is additionally supplied to an or switching logic device 221and to a second and switching logic device 198.

Summing device 153 has applied thereto the composite control signal fromthe A channel and the composite control signal from the M channel andthe diierence between such Signals is supplied over conductor 200 toamplifier 201 having its output :in turn supplied over conductor 202 tomemory device or flip op 203. Memory device 203 through conductor 204has its output supplied in one instance to the or switching logic device221 and through conductor 207 to the and switching logic device 196. Theoutput of the and switching logic device 196 is supplied over conductor220 to the or switching logic device 162. t

The summing device 154 has supplied thereto the B channel compositecontrol signal and the M channel composite control signal and anydifference between the signals as algebraically summed in supplied overconductor 210 to amplifier 211 having its output supplied over conductor212 to flip op device 213. The output from the bistable device 213 issupplied over conductor 214 to the or switching logic device 221. Device221 as represented has three inputs, and the output from the logicdevice 221 is transmitted overconductor 208 to the power switch 209which controls the circuit for an M light and also a master warninglight circuit of the aircraft.

Bistable device 213 which as stated has its output supplied overconductor 214 to the or switching logic device 221 also through asubconductor 216 has its output supplied to and switching logic device198. The output from the and switching logic device 198 is supplied byconductor 199 to the or switching logic device 179 which in turn throughconductor controls the power switch 181.

The switching logic devices as represented in FIGURE 2, as implied bythe terms applied to them, control the power switches 164, 181, 209. Theswitching logic or device 162 as represented in FIGURE 2 may be operatedeither from conductor 161 or conductor 220 whereas and switching logicdevice 196 will not be operated unless a control is suppliedconcurrently from conductors 194 and 207. Similarly the or switchinglogic device 221 willr supply an output if a control is supplied theretofrom either one of conductors (194, 197); (204, 206); or 214. Also theand switching logic device 198 will supply an output to conductor 199only if concurrently receiving a control from conductors 197 and 216.Also the or switching logic device 179 will supply an output if acontrol input is supplied thereto from either conductor 177 or conductor199.

It is noted that the bistable or memory devices 159, 174, 193, 203 and213 have pilot recycle controls 160, 175, 195, 205 and 215, to bedescribed, respectively to restore the initial conditions of thesebistable devices.

Also the power switches 164 :and 181 are provided with manual or pilotoverride controls 165, 184 to permit the pilot to override the switchinglogic and maintain the servomotors in the A channel or the servomotorsin the B channel operative to override any control supplied to conductor163 or 180.

summarizing the monitoring arrangement, the logic failure equationspertaining to FIGURE 2 Vit should be noted may be stated thus, :assumingthat the primed letters A', Bf, C', E', F', therein indicate the normalstate, unprimed letters the failed state: A channel failure=Ai (CE)wherein the plus sign is interpreted in the logic switching as Ior, thusgoing from A to A or going simultaneously from C and E to C :and E; Bchannel failure=B{-(CF); M channel failure=C+E-{F.

Reference is now made to FIGURES 3, 4 and 5 wherein the electricalschematic of the entire stabilization and monitoring arrangement isprovided. The description hereinafter will be concerned primarily withtne monitoring or fail safety provisions in the yaw stabilizationaugmentation system. This view or position is taken on the basis thatthe operation of the augmentation system of FIGURE 1 as thus describedis clear to one skilled iu the art in that deriving a yaw rate and alateral acceleration signal from AC pickofls, amplifying such signalsand converting them to two DC voltages and passing them through variousfilter networks and wherein such DC signals are used to control aservoampliiier that operates a series type hydraulic servomotor having afeedback to proportion its operation extent to the input signal,involves no new techniques.

However, a portion of the arrangement in FIGURE l pertinent to themonitoring apparatus and portions of FIGURE 2 intimately related to suchmonitoring provisions are shown in electrical schematic detail of somecomponents and described hereinafter. Further, inasmuch as the A, B andM channels shown in FIGURE l have duplicate sensors and similarcomponents, merely one of such channels will be described with respectto the detailed feature involved in the monitoring apparatus.

Referring to FIGURE 3 a yaw rate responsive device suc-h as a yaw rategyroscope 97 supplies an AC signal on conductor 98 to the diode limiter99 which limits the magnitude of the yaw signal supplied by conductor100 to the bridge amplifier 101. Amplifier 101 is shown in detail sinceportions thereof coact with the monitoring arrangement. Amplifier 101 isan AC amplifier having transistor elements instead of vacuum tubeelements therein. The monitoring arrangement is concerned with afeedback arrangement effected through transformer 220 with gain changingprovisions. The feedback arrangement comprises a feedback transformer220 having a primary winding 221 and secondary winding 222. The voltageon secondary winding 222 derived from the output of the amplifier is fedback through conductor 225 and various impedance paths to the basemember 224 o'f a transistor element 226. The gain of the amplifier 101is controlled by causing the feedback on conductor 225 to pass through ahigh or a low impedance path. This is achieved by selectively connectinga conductor 228 to ground through relay contacts 304. When the conductor228 is connected to ground, diode 223 is so biased as to be conducting,and the feedback path from conductor 225 is through diode 223, resistor231, resistor 232, conductor 233 to base element 224 of transistor 226.However, when the ground is removed from conductor 228, diode 223 isreverse biased and is at this. time essentially a high impedance or opencircuit so that the feedback path is through conductor 225, resistor230, conductor 233 to the base element 224 of transistor 226. Thus, withconductor 228 connected to ground whereby diode 223 is conducting thereis maximum feedback and consequently the amplifier has low gain but whenconductor 22S is removed from ground and the diode 223 is nonconductingthere is very little feedback and the amplifier has its highest gain.

The AC output from amplifier 101 is supplied to the AC to DC demodulator240 which may be of the vibrator type. The DC output from demodulator240 is supplied to gain control 106, thence to conductor 107, high passnetwork 108 such as a capacitor-resistor arrangement well known,aeroelastic network 110 as a resistor-capacitor arrangement to summingdevice 91. Summing device 91 is also supplied with a second controlsignal derived from lateral accelerorneter 112 supplying an AC signal onconductor 113 to the bridge amplifier 114 which may incorporate orinclude a demodulator similar to demodulator 240 above. The output ofsuch demodulator is supplied through gain control 118 having its outputsupplied to a lag network 120, 'which may be of the resistor-capacitortype, conductor 121 to algebraic summing device 91. Conductor 242extending from summer or summing device 91 transfers a composite controlsignal herein referred to as (Xu/A). While such composite control signalis used herein, it is clear that displacement type signals or othersignals may be utilized or a combination of such signals may be alsoused. The composite control signal from summing device 91 is transmittedby conductor 242 to the summing device 152 FIGURE 2. The application ofsuch control signal to a servoamplifier being considered understood fromabout, the application of the control signal to the monitoringarrangement will now be considered.

It is evident from FIGURE 2 that there are five corresponding buferamplifiers 157, 172, 191, 201, and 211 receiving control signals andcorresponding bistable or flip iiop devices 159, 174, 193, 203, and 213.Consequently due to the similarity in amplifiers and flip fiop devices,but one of the amplifiers and but one bistable device will be describedin detail in connection with FIGURE 4. FIGURE 4 will illustrate theupper amplifier 157 and bistable device 159 of FIGURE 2, and in so doingit is evident that a control signal for amplifier 157 is derived fromthe A left and A right servomotor displacements. The servomotors whenoperated in response to movements of their transfer valves in additionto operating the differential linkage arrangement also operate afollowup signal device Supplying an AC signal in accordance with thedirection and amount of displacement of said servomotors. These ACsignals are supplied to summing transformer 150 and thence by aconductor 243 to buffer amplifier 157. Amplifier 157 is a transistortype AC amplifier having two stages with class A operation. The bufferamplifier is redundant so that a single failure of the monitoringcircuitry will not render the monitoring arrangement inoperative. Theoutput of the amplifier 157 is supplied through a coupling transformeror output transformer 250 to bistable device 159. The bistable device isduplicated to increase reliability and but one of such memory devices orIbistable device will be shown and described in detail.

Bistable device or flip flop (FF) 159 utilizes transistors instead ofvacuum tubes and when power is initially applied to the memory orbistable device, the arrangement is such that transistor 255 will beconducting and transistor 256 will be off. In the overall systemoperation, when transistor 255 is conducting the automatic stabilizationsystem in consequence will be disengaged. Consequently, the bistabledevice 159 as is true of the other bistable devices must be cycled sothat transistor 256 is conducting and transistor 255 is nonconducting.This is achieved by applying a bias to the base of transistor 256. Suchbias on the base member of transistor 256 is supplied through a manuallyoperable recycle switch 260 connected to a 28 volt DC supply and in turnwhen closed connecting such supply through resistor 261, 262 to thetransistor ybase member as shown.

The output from memory device 159 is applied to tervrninal 264 FIGURE 5of a combined and/0r gate 265 having the 0r section 162 and the andsection 196. With respect to FIGURE 5 rather than FIGURE 2 in section196 there are and input terminals 273, 274. The or section inputterminals are 275 and 264. It is evident that both 273, 274 of the andsection 196 comprise an input of the or section 162. The output of theand/or switching logic 265 ris applied through conductor 285 to the-baseof a power transistor 287 which controls the engage circuit of theservos.

As in the case of the buffer amplifier and memory device, the and/ orgate and power transistor are redundant so that a single failure willnot disable the monitoring arrangement.

As stated, the primed letters indicate the normal state. Further todistinguish the inputs to the and/or gate 265 shown in detail in FIGURE5 from that merely shown in the block diagram FIGURE 2 as a redundantdevice,

the inputs have been given the subscript 1. Thus the normal or unfailedinputs to the and/or gate 265 are C1, E1; and A1. A'1 corresponds withthe output from i'p flop 159, C1 corresponds with the output of ip op193, and E1 is the output from flip flop 203. Similarly the and/or gate288 in the normal state supplies an output to power transistor 289. Thusin the normal state, and with selectively operable switch 290 in closedposition, a circuit is completed from the +28 volt source, switch 290,relay winding 291, through the power transistors 287, 289 to ground toenergize relay Winding 291.

Energization of relay winding 291 raises relay armatures or movablecontactors 293, 294, 295. As operated, movable contact 293 closes acircuit from the 28 volt source through a winding 296 corresponding tothe operating means 66 of the solenoid valve 65 in the aforesaid Patent2,952,424 to Schurr. Similarly operation of relay arm 294 closes acircuit through a winding 297 of the A left rudder servomotor solenoidvalve corresponding to the valve of the A right rudder servomotor above.Thus` in normal operation, the left and right rudder servomotors of theA channel or section of the stabilization system are in condition oreffective for operation under the control of their transfer valves inturn controlled by the signal section of FIGURE 3. Additionally uponoperation of relay arm 295 a circuit is completed from the 28 voltsupply to switch 290, relay arm 295, closed switch arm 300, relaywinding 303, to ground effecting operation of relay arm 304 connectingconductor 228 of the amplifier shown in FIGURE 3 to signal groundthereby maintaining the normal or reduced Vgain of amplifier 225.

If we consider the engage equation of FIGURE to be FA=A'1C'1|A1E1 whereFA represents the engagement of channel A and plus indicates an orcondition then it is apparent that if we have in the normal Al, for eX-ample no output on 161, along with either Cl or E'l the system willremain engaged; however, if Al is locking then even if C1 and E1 bepresent the system will not remain engaged. Stating it in the dsengageform of the equation FA=A1+C1E1, if A'l changes to A1 the system will bedisengaged or if C1 and E1 are simultaneously changed to C1 and E1 thesystem will be disengaged.

The yaw logic override switch 305 which is provided for theA and Bchannels, as indicated in FIGURE 5 by the pertinent indicia, may beoperated to apply a 28 volt bias to the base members of the powertransistors 287, 289 to maintain the system engaged.

While merely the A channel of the yaw stabilization System has beendescribed in detail, it is evident that a similar arrangement would beprovided for the B channel for 4controlling the power switch 181 in amanner similar to which the power switch 164 of the A channel iscontrolled. Similarly from the above a monitoring arrangement for powerswitch 209 of FIGURE 2 relating to the M or monitoring channel tocontrol various light circuits would be provided.

When initially engaging the yaw stabilization system, it is evident thatthe left and right A channel servomotors 50, 60 have similardisplacements since they are spring centered on disengagement so thatthe output of summer 150 FIGURE 2 will be zero. Therefore the fiip flop159 after being recycled to zero by the pilot as by switch 260 FIG- URE4 has a normal output A.

If the control signals from the ight condition sensors of the A channeland the B channel are the same, there will be no signal input to flipflop 193 which will have, after being recycled, a normal output C. Ifthe signal output of the A channel and the M channel from theirrespective ight condition sensors are the same, there will similarly bea normal output E from flip flop 203, and at this time closing of switch290 FIGURE 5fwill result in energization of power switch 164 and theenergization of the solenoid windings 296, 297 engaging or renderingeffective for operation the left and right rudder servomotors. Furthercontrol or operation is thereafter 10 provided by their respective pilotvalves or transfer valves as in the Schurr Patent 2,952,424. If otherthan 'a normal state output is supplied from flip op 159 FIGURE 2 to theor gate 162 the system will remain unengaged or will be disengaged ifpreviously engaged.

Further explanation of FIGURE 5 relating to the and/ 0r gate 265 that iscomprised of or gate 162 and and gate 196, in FIGURE 2, perhaps isdesirable. FIGURE 2 is set up on the logic failure equation for the Achannel: failure equals A-l-CE which interpreted means that a failurewill occur upon the presence of A instead of A'. Failure will also occurupon the simultaneous occurrence of C instead of C' along with E insteadof E. However failure will not occur 'if A is present along with eitherC or E' present.

In FIGURE 5 (considering the electric schematic and its redundantcounterpart, 288 in FIGURE 5) it is apparent that diodes 310 willconduct if either Cl or E1 be present on terminals 273, 274 and thusdiodes 310 are the or gate diodes. Further, the conduction throughdiodes 311 depends upon A1 and the condition of C' and E'. Thereforediodes 311 constitute the and gate. Consequently if A instead of A'1 isapplied to terminal 264 the and gate comprising diodes 311 will not beeffective. However, if A1 indicating no error be applied to terminal 264and if either Cl or El indicating no error to one terminal be applied toterminal 273 or 274, diodes 311 will be effective and the and gate isoperative to retain the stabilization system engaged.

It will now be evident that there has been provided a novel monitoringand interlock arrangement for an automatic yaw stabilization systemwhich upon the presence of a differential in operation of a left andright servomotor will effect the disengagement or render ineffective thefurther operation of the two rudder surfaces from their respectiveservomotors except from automatic centering and wherein such monitoringarrangement, control signals derived from aircraft flight response arecompared so that if one signal device arrangement out of three signaldevice arrangements sensing' a similar fiight condition has a differentresponse, the two servomotors will also be rendered ineffective. Whenthe servomotors are thus disengaged or rendered ineffective, two otherservomotors operating the same surface have the gain in their controlsystems automatically increased to provide similar operation of eachrudder surface as if the two servomotors connected to each surface wereoperating as normal.

Although but one embodiment of the invention has been illustrated anddescribed in detail, it is to be understood that the invention is notlimited thereto and that various changes can be made in he design andarrangement of the parts without departing from the spirit and scope ofthe invention except as limited by the subjoined claims.

I claim:

1. In an aircraft having a control system, a pair of automaticstabilization units, each of said units including first electrical meansfor supplying a signal indicating the rate and direction of turn of theaircraft about a reference axis, actuator mechanism adapted foractuating the control system in response to said signal from said firstelectrical means so as to stabilize the aircraft about its referenceaxis a further electrical means for supplying a signal indicating therate and direction of turn of the craft about the reference axis, secondelectrical means interconnecting said pair of stabilization units andsaid further electrical means for sensing a'failure in one of said unitsby comparing signals of the electrical means of the two units andfurther electrical means and for modifying said first electrical meansin the other of said units to increase said signal therefrom forincreasing the actuation of said actuator mechanism associatedtherewith, whereby the loss of motion of the actuator mechanism in thefailed unit is compensated by the increased motion of the actuatormechanism of the operating unit.

2. In an aircraft having a control system, a pair of automaticstabilization units, each of said units including first electrical meansrfor supplying a signal indicating a change in a flight condition of theaircraft from a normal ight condition, actuator mechanism adapted foractuating the control system in response to said signal from said firstelectrical means and providing actuator position feedback signal to saidrst electrical means so as to stabilize the aircraft about its normalcondition, second electrical means interconnecting -said pair ofstabilization units also receiving and comparing said feedback signalsVing first means for supplying a signal in response to the rate anddirection of change in a first flight condition from normal and a changein a second flight condition from a normal condition, actuator mechanismadapted for actuating the control system in response to said signal fromsaid iirst means so as to stabilize the aircraft with respect to saidnormal conditions and providing an actuator position signal7 secondmeans interconnecting said pair of stabilization units and additionallyreceivingrand comparing the actuator position signals for sensing afailure in one of said units and for modifying said first means in theother of said units to increase said signal therefrom for increasing theactuation of said actuator mechanism associated therewith, whereby theloss of motion of the actuator mechanism in the failed unit is.compensated by the increased motion of the actuator mechanism of theoperating unit.

4. In an aircraft having a control system, a pair of automaticstabilization units, each of said units including first electrical meansfor-supplying a signal indicating the rate and direction of turn of theaircraft about a reference axis, actuator mechanism adapted foractuating the control system in response to said signal from said firstelectrical means so as to stabilize the aircraft about its referenceaxis said actuator mechanism supplying a feedback signal to the rstelectrical means, second electrical means also receiving said feedbacksignals and thus interconnecting said pair of stabilization units forcomparing said feedback signals for sensing an electrical failure in oneof said units and for modifying said first electrical means in the otherof said units to increase said signal therefrom -for increasing theactuation of said actuator mechanism associated therewith, and meanscontrolled by the second electrical means rendering the actuator meansineffective in the failed unit.

5. In automatic condition control apparatus, three sensing devices eachresponsive to the same condition and each providing a signal, combiningmeans receiving said signals and forming two different pairs of signalsfrom the signals from the three devices, and separately summing the twosignals in each pair, further means connected to the control apparatusand jointly responsive to the combined signals from both pairs andeffective on any difference of the three signals causing inequality ofthe two signals in each pair rendering the control apparatus ineffectiveto control the condition.

6. In control apparatus for an aircraft having a plurality of similarcontrol surfaces each having a separate servomotor operating it: aplurality of sources of control signals; means operated by one signalsource operating one motor; means operated by another signal sourceoperating another motor whereby any motor is controlled by one signalsource; monitoring means comparing among themselves the signals from allsignal sources; and means controlled by the monitoring means upon thedetermination of an unlike signal from one signal source among theplurality of signals, rendering the servomotor of such signal sourceineffective to `control said surface.

7. The apparatus of claim 6, wherein the monitoring means is responsiveto a number of signal sources exceeding by one the number ofservomotors.

8. The apparatus of claim 6, wherein said monitoring means includes anarrangement that maintains said servomotor ineffective to control saidsurface despite subsequent agreement of said one signal with othersignals compared therewith.

9. In condition control apparatus, three operable devices eachresponsive to the same condition and each providing a signal, combiningmeans forming two different pairs of signals from the three sensingdevices and separately summing two signals in each pair, further meansjointly responsive to the combined signals from both pairs and effectiveon any difference of one signal from the other two of the three signalsproviding an output.

10. In ight control apparatus, three operable devices each responsive tothe same flight condition and each providing an electrical signal,signal combining means forming two different pairs of signals yfrom thesignals from the three signal devices and separately summing two signalsin each pair, and an and circuit jointly responsive to concurrentresultant signals from both pairs whereby on any difference of onesignal with respect to the other two signals an output is supplied bythe and circuit.

References Cited UNITED STATES PATENTS 2,609,165 9/1952 Hill 244-422,658,701 11/ 1953 Robertson 244-42 3,156,855 11/1964 Righton et al.244-77 FERGUS S. MIDDLETON, Primary Examiner.

